jb wrote:
* Is it possible to apply a video effect only to a (spatial) part of
  a clip ? The area can be restricted to a rectangle. Is it what the
  "region" transition of mlt is for?

Yes, that is probably a much wanted feature, but it has not been implemented 
in Kdenlive yet.
The obscure effect can be applied to only a rectangular area of the clip.  Is it possible to simply copy that mechanism to other effects or is my assumption way too simple?